I've seen this type of markup produced by in-browser WYSIWYG editors that don't inspect the existing DOM/markup for redundancy when applying various styling commands.
In some dramatic cases, you would see alternating re-wrappings of (strong,bold,strong,bold,strong,bold) tags as users who were using different browsers (which produced slightly different markup for various style commands) would re-bold each others' sections of text from day to day.
In some dramatic cases, you would see alternating re-wrappings of (strong,bold,strong,bold,strong,bold) tags as users who were using different browsers (which produced slightly different markup for various style commands) would re-bold each others' sections of text from day to day.